In the data storage field, microfilm has found widespread use as an inexpensive, permanent storage medium for a variety of data. When used for storing large quantities of digital data, microfilm typically records bits of digital data, such as binary ones and zeros, as black and white images confined to predetermined film areas called "data cells." Photographic recording processes often format the digital data into a compact array of rows and columns.
Image processors retrieve recorded digital data from microfilm by first scanning the film with an optical sensor. Such scanning generates digital strings of grayscale image data representing, for each sensor pixel, several levels of luminescence ranging from black to white; for example, an 8-bit byte associated with a sensor pixel can represent black, white, and 254 shades of gray. Thereafter, the image processor converts the scanned grayscale image data into binary image data through a thresholding process that, for example, may represent all grayscale image data above a certain level as a binary one and below that level as a binary zero. After thresholding, the image processor converts the binary image data into an output stream of digital data, which represents the original digital data stored on the film.
Additional detailed discussions regarding systems that are designed to retrieve digital data stored on film may be found in my copending, commonly assigned U.S. patent application, entitled "System and Method for Bidirectional Adaptive Thresholding," Ser. No. 07/797,630; filed Nov. 25, 1991, which is incorporated by reference herein. Descriptions of prior art systems for recording or reading digital data on film also appear in the following U.S patents issued to Russell (hereinafter collectively referred to as the "Russell" patents): U.S. Pat. Nos. 3,501,586 (issued Mar. 17, 1970); 3,624,284 (issued Nov. 30, 1971); 3,795,902 (issued Mar. 5, 1974) and 3,806,643 (issued Apr. 23, 1974); and in U.S. Pat. No. 4,603,414 (issued to Ackerman et al on Jul. 29, 1986).
In order to retrieve digital data from film, the digital data must be accurately sampled at proper locations. Usually, the data must be sampled at specific pixel positions. This often means that the sampling system must be accurately synchronized to the film. Various prior art systems, such as those described in the Russell patents, achieve this synchronization by using a conventional phase-locked loop to adjust the speed of the film or the frequency of an oscillator. In any case, maintaining bit-to-bit synchronization will normally require that various scanning parameters be continuously adjusted in order to vary both sampling phase and sampling pitch.
Although prior art systems have served the purpose, for several reasons, they have not proved entirely satisfactory in maintaining bit-to-bit synchronization under all conditions of service. First, many prior art systems do not adjust for gradual changes in data-cell size and spacing. Such changes often result from changes in magnification that are inherent in and over time occur in most conventional optical scanners. Second, other prior art systems cannot compensate for growth and shrinkage of exposed areas of the film, i.e., actual changes in data-cell size across a scan line. In this regard, conventional photographic processes often create slight variations in the sizes of the exposed film areas during data recording. Third, still other prior art systems cannot accurately retrieve data from film marred or disfigured by dirt, scratches, fingerprints, or other artifacts. Marred or disfigured film areas may introduce false black-white transitions, or they may move or erase existing transitions.